1. Technical Field
This invention relates to rotor blades having utility in a turbine engine in general, and to methods of repairing rotor blades used in gas turbine engines in particular.
2. Background Information
Rotor assemblies are utilized in the fan, compressor, and turbine sections of a gas turbine engine. Each rotor assembly includes a plurality of rotor blades distributed around the circumference of a disk. In a conventional rotor assembly, the rotor blades are attached to the disk by mechanical means such as a xe2x80x9cfir-treexe2x80x9d type attachment where a fir-tree shaped blade root is received within a mating recess in the disk. Alternatively, the rotor blades can be integrally attached to the disk by metallurgical attachment or be machined from a forging. The resultant integrally bladed rotor (IBR) obviates the need for blade attachment hardware and the weight associated therewith. IBR""s also permit the use of a rotor disk smaller, and therefore lighter, than would be acceptable in a comparable rotor assembly having a mechanical attachment scheme.
Because of the considerable cost and time incurred in the manufacture of a rotor blade, it is preferable to repair rotor blades when possible rather than replace them. This is particularly true in the case of an IBR. Repair of a rotor blade airfoil is typically necessitated by wear that normally occurs during operation. Airfoil repairs can also be necessitated, however, by foreign object damage (FOD) that occurs when a foreign object strikes an airfoil. If the extent of the wear or damage (hereinafter referred to generically as damage) is below a predetermined threshold, it may be possible to xe2x80x9cblendxe2x80x9d the area of the airfoil back to within acceptable standards. Blending usually involves machining the damaged area back to within acceptable tolerances. If the damage exceeds the blending threshold, conventional practice dictates that the damage be evaluated to determine the appropriateness of a xe2x80x9cpatchxe2x80x9d repair wherein the damaged portion of the airfoil is removed and replaced.
Anytime a portion of an airfoil is removed and replaced, the repair must not compromise the integrity of the airfoil. The useful life of an airfoil (an indicator of integrity) is typically determined by evaluating the airfoil for existing stresses or those experienced during operation. Stresses experienced during operation will regionally vary within the airfoil as a function of rotational speed, operating environment, etc. This is particularly true in certain types of airfoils; e.g., low aspect ratio airfoils where the length over width ratio is approximately two (2) or less. The principal types of stresses within an airfoil experienced during operation can be described in terms of steady-state stresses and vibratory related stresses. Steady-state stresses are a function of centrifugal loading, gas pressure, and thermal gradients and can be considered a constant for purposes of determining acceptable stress limits within a region of an airfoil at any given rotational speed. Vibratory related stress, in contrast, is a function of the environment in which the airfoil operates. An airfoil used within a gas turbine engine is subjected to periodic and non-periodic excitations present within the environment, and the excitations collectively produce the vibratory related stresses. Periodic excitations can be problematic if they coincide with a natural frequency of the airfoil. The resonant condition that results from the coincidence of the frequencies can result in undesirable oscillatory displacements that produce periodic-type vibratory related stresses. Non-periodic vibration responses such as xe2x80x9cflutterxe2x80x9d or xe2x80x9cbuffetingxe2x80x9d are independent of the resonant frequency(ies) of the airfoil. Flutter, for example, is a function of aerodynamic damping. If aerodynamic forces acting on an airfoil are such that energy is added to rather than dissipated from the airfoil, the additional energy can cause non-periodic displacement of a portion of the airfoil (i.e., xe2x80x9cflutterxe2x80x9d) that causes the airfoil to experience non-periodic-type vibratory related stress.
Another type of stress that must be considered in a patch repair is the localized residual stresses that are created when the patch is bonded to the airfoil. Bonding processes such as welding typically impart considerable thermal energy into the substrates to be joined. The rate at which the thermal energy is removed from the substrates during and after the joining process is generally not uniform (e.g., exterior surface regions cool at a faster rate than interior regions), consequently precipitating the formation of residual tensile stresses in the area of the joined substrate subjected to the thermal energy. The additional stress attributable to the joining process is additive to the vibratory and steady-state stresses discussed above. Hence, there is value in minimizing the length of a bond joint. This is particularly true in airfoil applications where a variety of vibratory modes exist and the bond line is likely to extend across multiple node lines and thereby extend through a variety of regions subjected to different stresses. There are mechanisms available to reduce stress along a bond line such as peening and/or heat treatment. Stress reducing steps add cost to the repair and in some instances it may not be possible to limit the effects of the stress reducing process to the regions desired.
In short, airfoils utilized within a gas turbine engine typically experience a variety of different stresses generally describable in terms of steady-state and dynamic stresses. In many instances, therefore, it is not enough to consider only those stresses that relate to modes of vibration associated with the natural frequencies of the airfoil, and it would be advantageous to have a method for repairing rotor blades that accounts for all of the stresses that an airfoil will typically experience during operation, including steady-state stresses, vibratory related stresses, and residual stresses.
It is, therefore, an object of the present invention to provide a method of repairing a rotor blade that accounts for the multiple types of stresses the airfoil typically experience during operation.
It is another object of the present invention to provide a repair method that can be standardized wholly or in part to facilitate the repair process.
According to the present invention, a method of repairing an airfoil is provided that includes the steps of: (a) determining repair regions of the airfoil that are likely to be damaged during a period of operation; (b) creating a stress profile for the airfoil that considers dynamic, steady-state, and residual stresses; (c) selecting a replacement section patch line using the stress profile and the determination of those regions likely to be damaged during operation; (d) providing an airfoil replacement section with a predetermined shape having a bond surface that substantially mates with the patch line; (e) removing a portion of the airfoil up to the patch line; (f) bonding the airfoil replacement section to the airfoil along the patch line; and (g) shaping the patched airfoil.
The determination of which airfoil regions are likely to be damaged (xe2x80x9cdamage regionsxe2x80x9d) during a period of operation is made using a predictive method. Empirical data that can be used to predict damage may be collected from numerous sources including similar existing engines, test engines of the same type, or in-service engines of the same type. Collectively, empirical data provides a reliable mechanism for predicting which areas of an airfoil are likely to be damaged, the type of damage, and the nature of the required repair. An empirically derived damage map provides a means for evaluating a potential patch line. Other predictive methods may be used alternatively to derive damage maps.
A stress profile is the product of determining the stresses that the airfoil will likely experience during operation including steady-state stresses, dynamic stresses, and residual stresses present from joining processes. Steady-state stresses may be defined as those resulting from centrifugal loading, gas pressure loading, and thermal gradients. Dynamic loading involves both periodic and non-periodic loading. Periodic loading describes those loads that the airfoil experiences xe2x80x9cnxe2x80x9d number of times during a rotation of the rotor assembly. Non-periodic loading encompasses all remaining dynamic loadings, such as flutter and buffeting.. In all cases, the stresses attributable to the various influences are collectively mapped as a function of position on the airfoil and rotational speed, collectively forming the stress profile.
The patch joint line is selected using both the predicted damage map and the stress profile. As a first step, a particular damage region on an airfoil might be selected as a candidate for a patch repair. Potential patch lines for that region are evaluated for stresses using the stress profile. In some instances, in may not be possible to use a proposed patch line because stresses encountered at some point along the patch line exceed an acceptable stress level. In that case, if it is possible to select a new patch line for the same damage region, the evaluation process is repeated for the new patch line. If, on the other hand, it is not possible to define an acceptable patch line for a given region, the region may have to be redefined using the predicted damage map (e.g., capture 75% of the damage region rather than 85%) and the process repeated for the redefined region until an acceptable patch line is determined.
One of the advantages of the present method is that it provides an airfoil repair that considers multiple stress types rather than just dynamic stresses attributable to periodic-type vibrations. Non-periodic dynamic loading and/or steady-state loading in many cases causes significant stresses that may be equal to or greater than those attributable to periodic loading. In those cases, positioning a patch line along or adjacent a natural frequency node line may run afoul of high stress regions associated with non-periodic modes of vibration like flutter and/or high stress regions attributable to steady-state loading. The present method also addresses different stresses typically associated with particular areas of an airfoil. For example high cycle fatigue can cause an outer airfoil corner to fail under certain circumstances. In that instance, the highest stress associated with the failure would be at the failure line, not at the corner itself where the displacement is the greatest. A repair method that equates stress with displacement may not provide a viable repair in that instance.
Another advantage of the present invention is that a patch repair is provided that can be used to increase the useful life of a rotor blade. The empirically derived or otherwise predicted damage data used under the present method to select a patch line enables the person developing the repair to select a patch line that encompasses as much of an anticipated damage region as is possible without unnecessarily increasing the length of the patch joint. As a result, the number of damage areas likely to be repairable by a particular patch is greater and the useful life of the airfoil likely extended.
Another advantage of the present invention is that a repair method is provided that allows for standardization without having to include unnecessary portions of the airfoil. Some prior art patch repairs disclose an all-encompassing patch sized to ensure that all potential damaged portions of a blade are replaced by a single patch. Such a patch is likely to include portions of the airfoil less apt to be damaged than others. The present method uses predicted damage data and a stress profile to judiciously define a patch geometry that captures the desired damage region(s).
A significant benefit of the present method lies in the cost of the replacement section. Using an entire airfoil as a replacement section, or even a substantially sized replacement section will drastically increase the cost of the repair thereby minimizing the economic benefit of the repair. Larger replacement sections are likely to have greater geometric complexity and consequent manufacturing cost. An advantage of the present invention is its ability to judiciously determine an optimum replacement section.
A judiciously determined optimum replacement section is also likely to require a relatively short length joint. Because any joining process will have some inherent probability of defect inclusion within the joint region, the ability of the present method to provide a repair with a relatively short length joint is a further advantage.
These and other objects, features and advantages of the present invention will become apparent in light of the detailed description of the best mode embodiment thereof, as illustrated in the accompanying drawings.